Color input signals of color display devices (abbreviated display devices hereinafter) such as liquid crystal display (LCD) devices or conventional cathode ray tube (CRT) display devices consist of digital signals of three original colored lights that are red (R), green (G) and blue (B) (called R,G,B hereinafter). Input and output are indicated by scales of 0–255. Applying color light mixing principle, based on the three original color lights and ratio control of different luminance, millions of different readings may be obtained. The color temperature relates to the energy dispersion relationship of the R,G,B color lights. The display device can radiate different color temperatures through firmware by setting the R,G,B output bases. These bases are fixed once the firmware is written and hardwired. According to physics, the color temperature represents the spectrum characteristics of an illuminant. When the spectrum distribution of an illuminant is same as the radiation of a black body, the absolute temperature (K°) corresponding to the black body radiation is called the color temperature of the illuminant. There are several preset color temperatures for the general display devices, such as 9300 K°, 6500 K°, 5000 K°, etc. They generally are indicated by D93, D65, D50.
Because of different output and input devices, digital image data may be unchanged, but different color display methods can generate different display results. This makes color reproduction a big challenge. For instance, in the color reproduction printing based on three colors C (cyan), M (magenta) and Y (yellow), how can an image file achieve the same printed results through different types of printing facilities? How can it display the same color on different brands of display devices (based on RGB for color reproduction)?
To resolve these problems of different facilities or color reproduction, a color profile mode must be defined and remain the same when different devices are used. It is called the standard reference color gamut. The RGB color or CMY color of different devices can find a corresponding color in the standard reference color gamut so that color conversion among different devices may be maintained without distortion. In 1931, Commission Intornation De'l E'clairage (CIE) or ICI (International Commission on Illumination) published a physical measurement method that uses numbers to quantify color space of various colors. For instance, CIE-R.G.B coordinates (CIE1932 standard chromaticity observer) were generated by matching color and experiments. CIE L*a*b* and CIE1931 Yxy color representation methods are the frequently used color spaces. The color management system (CMS) of different devices require a color profile defined by accurate color reproduction characteristics to meet the color conversion requirements of different display devices. The color profile generally is stored in each display device through a firmware. The techniques for generating color profile can be found in prior arts such as U.S. Pat. No. 5,619,427.
Color is a reflection of light sensed by human eye observation. With illuminants of different wavelengths or different color temperature, even the display device of the same RGB output bases will give human eyes different color sense.
The technique for calibrating color temperature of the display device based on the color temperature of the environmental illuminant can be found in U.S. Pat. No. 5,791,781 entitled “Method of determining color temperature for color display device”. It mainly uses two different patch sets to determine the color temperature of the display device. One set consists of a plurality of patches that displays gray scale in various illuminant elements. Another set consists of a plurality pairs of patches of the same color in different spectrums. This technique, after taking into account environmental illuminant conditions, can determine the color characteristic data of the color temperature of a display device in the CMS mode and provides required color temperature information to generate the required color profile without using the colorimeter. However, it does not provide any suggestions to ensure that the color temperature radiated from the display device can conform to standards.
The known methods for calibrating the color temperature of display devices mostly are through the functions provided by hardware and firmware (such as the firmware in the display device that stores the color profile and a calibration button), or through software (such as the image editing software Photoshop). In fact, every set of display devices manufactured by the same vendor in the same year has its own color gamut because of the different RGB phosphorescent agents used. Hence, it cannot be guaranteed that the radiated color temperature of the display devices of the same brand or model that use the same color profile can reach the standard color temperature.
The hardware functions of a good display device allow users to calibrate the color temperature by themselves. It is even possible to adjust the individual RGB intensity. However, for a display device that does not offer color-calibrating functions, calibration of the color temperature has to be accomplished through software. For instance, for a display device which has color temperature 9300 K° (blue-oriented ), to calibrate to 5000 K° (red-oriented ),the calibration method is to reduce the output base of the blue light (B). Hence when input of blue is 255, maximum output of green color is 235, and output of blue color should be no greater than 219. With the green and blue color reduced, the display device is red-oriented and can reach the desired color temperature. Although the color temperature reaches the desired goal, maximum output of the green and blue light decreases. As the luminance is not calibrated in a corresponding manner, total light intensity and color diminish.